This invention relates to a method of charging a secondary battery and a charging apparatus for using such a method.
A secondary battery has been used for various portable electronic instruments such as a cellular phone, a note type personal computer, a video camera and so on. Of late, the secondary battery is also spotlighted as a power supply for a vehicle such as a bicycle or a car.
There have been used various types of a secondary battery such as a lead-acid storage battery used most anciently and a nickel-cadmium (Ni-Cd) battery, a nickel-hydrogen (Ni-MH) battery, a lithium-ion (Li-Ion) battery which are developed lately and so on. A Ni-Cd battery or a Ni-MH battery among the aforementioned secondary batteries have had an overwhelming market share for the portable electronic instruments because they can be compacted and can replace the conventional manganese dry battery.
However, the conventional secondary batteries disadvantageously have a charging time of 2 through 5 hours normally required as they are charged by the conventional charging method recommended by the battery maker. There has been provided by a maker other than the battery maker a so-called quick charger which is able to charge the secondary battery for a charging time of about 30 minutes to one hour with a higher charging current. Such a quick charger causes the secondary battery to have a higher temperature at the end of charging and to have a shorter battery life due to its higher charging current. In addition thereto, a gas generates within the battery so as to make the inner pressure higher, if any, which disadvantageously causes the explosion of the battery to be made.
Furthermore, as the secondary battery repeats the incomplete charge and discharge, there happens a so-called memory effect of the secondary battery which causes the effective service capacity to gradually decrease. This disadvantageously causes the secondary battery to be unable to be used earlier. The memory effect of the secondary battery is caused by making a so-called addition charge which is made by charging the battery before the capacity of the secondary battery is completely consumed or discharged. In order to decrease the memory effect of the secondary battery, it should be completely discharged before it is charged. However, this causes the charge of the secondary battery to take a substantially longer time and the operation to be made troublesome. For example, in case that the cellular phone should be charged before going outdoors, the addition charge will be inevitably made because there has no enough time.
The memory effect of the secondary battery is caused by a chemical material being crystallized due to its electrochemical change. The once generated crystal can be effectively broken by activating the chemical material while a higher electric field is applied thereto or a higher voltage is applied thereto so that a larger charging current flows. To this end, there has been proposed a method of preventing the memory effect of the secondary battery by a large charging current or an impulse pulse current flowing through the secondary battery whereby the memory effect is prevented. However, as aforementioned, as the charging current is made larger, the battery will be fatally damaged due Lo a higher temperature in the interior of the battery and a gas generation.